1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device.
2. Description of the Related Art
A charge trap type nonvolatile semiconductor memory has been proposed which uses a charge storage insulating film for charge trapping as a charge storage layer (see Jpn. Pat. Appln. KOKAI Publication No. 2004-158810). In the charge trap type nonvolatile semiconductor memory, charges injected into the charge storage insulating film through a tunnel insulating film are trapped in trap states in the charge storage insulating film. The charges are thus stored in the charge storage insulating film. A typical charge trap type nonvolatile semiconductor memory is of a MONOS or SONOS type. A silicon nitride film or the like is used as a material for the charge storage insulating film.
In the charge trap type nonvolatile semiconductor memory, the charge storage insulating film is normally formed in the area between adjacent memory cells in a bit line direction. The charge storage insulating film has a high charge trap state density. Thus, a decrease in the distance between the adjacent memory cells may disadvantageously cause the charges stored in the charge storage insulating film to migrate between the adjacent memory cells. As a result, the operation characteristics and reliability of the semiconductor device may disadvantageously be degraded.
Another charge trap type nonvolatile semiconductor memory has been proposed which uses a tunnel insulating film with an ONO (silicon oxide film/silicon nitride film/silicon oxide film) structure (see Jpn. Pat. Appln. KOKAI Publication No. 2007-184380). However, because of the high charge trap state density of the silicon nitride film, a decrease in the distance between adjacent memory cells may disadvantageously cause charges trapped in the silicon nitride film to migrate between the adjacent memory cells. As a result, the operation characteristics and reliability of the semiconductor device may disadvantageously be degraded. These problems may commonly occur when a tunnel insulating film of a stack structure is used in which an insulating film with a high charge trap state density is provided between insulating films with a low charge trap state density.
Thus, in the conventional art, the migration of charges between the adjacent cells may disadvantageously degrade the operation characteristics and reliability of the semiconductor device.